Currently, aluminum metal is commercially manufactured in two steps. The first step involves extracting alumina (Al.sub.2 O.sub.3) from bauxite using a Bayer process. The second step involves reducing the alumina which is dissolved in a mixture of molten cryolite (Na.sub.3 AlF.sub.6) and aluminum trifluoride (AlF.sub.3) in an electrolysis cell at about 950 to 960.degree. C. using the Hall-Heroult process. In the second step, aluminum-containing ions are reduced electrochemically to produce metallic aluminum at the metal cathode surface.
During normal electrolysis, carbon anodes are consumed as oxygen-containing ions react with the carbon anodes to form carbon dioxide and aluminum metal according to the following equation: EQU 2Al.sub.2 O.sub.3 +3C.fwdarw.4Al+3CO.sub.2
However, at certain conditions, the electrolysis cell approaches an anode effect. This approach is characterized by the following events:
(1) The alumina concentration in the bulk of the electrolyte decreases below 2% by weight; PA1 (2) Higher concentrations of fluoride ions prevail near the anode as the concentration of oxygen containing ions decreases; PA1 (3) The anode polarization voltage increases significantly; PA1 (4) The critical current density of the carbon anode is exceeded for the discharge of only oxygen-containing anions; and PA1 (5) F.sub.2 is eventually discharged at the anode surface from the decomposition of cryolite. PA1 (a) removing inorganic fluorides from a vent gas comprising inorganic fluorides and at least one of CF.sub.4 and C.sub.2 F.sub.6 to obtain a purified vent gas; and PA1 (b) contacting the purified vent gas with a membrane at conditions effective to obtain a retentate stream rich in at least one of CF.sub.4 and C.sub.2 F.sub.6, and a permeate stream depleted in at least one of CF.sub.4 and C.sub.2 F.sub.6. PA1 (a) electrolytically reducing alumina dissolved in a mixture of molten cryolite and aluminum trifluoride in an electrolysis cell to produce aluminum; PA1 (b) withdrawing a vent gas comprising F.sub.2, HF, and at least one of CF.sub.4 and C.sub.2 F.sub.6 from the electrolysis cell; PA1 (c) contacting the vent gas with alumina at conditions effective to react F.sub.2 and HF with the alumina to produce aluminum trifluoride and a gas stream comprising the at least one of CF.sub.4 and C.sub.2 F.sub.6 ; PA1 (d) recycling at least a portion of the aluminum trifluoride from step (c) to the electrolysis cell; and PA1 (e) contacting the gas stream comprising the at least one of CF.sub.4 and C.sub.2 F.sub.6 with a membrane at conditions effective to obtain a retentate stream rich in at least one of CF.sub.4 and C.sub.2 F.sub.6, and a permeate stream depleted in at least one of CF.sub.4 and C.sub.2 F.sub.6.
During anode effects, the fluorine discharged at the anode reacts with the carbon to form CF.sub.4 and C.sub.2 F.sub.6 according to the following equation: EQU 2Na.sub.3 AlF.sub.6 +2C.fwdarw.2Al+2NaF+CF.sub.4 +C.sub.2 F.sub.6
For a more detailed discussion of the anode effect, see Alton T. Tabereaux, Anode Effects, PFCs, Global Warming, and the Aluminum Industry, JOM, pp. 30-34 (November 1994).
For a typical electrolysis cell, the emission rate of CF.sub.4 and C.sub.2 F.sub.6 per day is 0.25 kg. There are normally 100 to 200 cells per plant. Therefore, the daily emission of CF.sub.4 and C.sub.2 F.sub.6 per plant is about 50 kg.
The emission of CF.sub.4 and C.sub.2 F.sub.6 from aluminum plants has typically been vented directly into the atmosphere. However, these gases, which are 10,000 times more potent than CO.sub.2, have recently been classified as global warming gases. Thus, with the signing of the United Nations Framework Convention on Climate Change which is aimed at reducing the emission of global warming gases, there is a significant need in the industry for a way to minimize or eliminate the emission of these gases into the atmosphere.
Accordingly, it is an object of the present invention to address this need in the aluminum industry.
These and other objects of the invention will become apparent in light of the following specification, and the appended drawings and claims.